Plasma CVD is widely used to form thin films in the manufacture of semiconductor devices. In plasma CVD, a gaseous fluoro compound such as NF3 is ionized, and the resulting ionized species are used for etching the surfaces of semiconductor substrates or cleaning CVD chambers. During such plasma ionization, F2 gas is also generated, but F2 gas is treated as exhaust gas.
For example, a method has been known for ionizing F2 gas and using the resulting ionized species to clean the inner wall of a CVD chamber (e.g., see patent document 1), but this method uses a plasma of F2 gas rather than F2 gas itself.
Surface treatment methods for materials other than semiconductors such as metal materials and plastic materials using a plasma derived from a fluoro compound have been reported (see patent document 2, and patent document 4). However, the plasma may penetrate into the article to be surface-treated to damage the article in these methods.
A surface treatment method using unionized F2 gas has also been reported (e.g., see patent document 3). However, the method directly using F2 gas has the following problems. First, it is difficult to store, transport and use a large amount of F2 gas because of the toxicity, hazardousness, reactivity, and corrosiveness of F2 gas. In order to reduce hazardousness, diluted F2 gas can be used, but costs for storage and transportation increase. Moreover, very stable materials must be used for the sites exposed to F2 gas, which add costs of the surface treatment equipment.
A method for treating the surface of an article comprising exciting a halogen compound by applying a low-frequency voltage on it at or near atmospheric pressure to decompose it and generate a halogen or hydrogen halide has also been proposed (see patent document 5). This method has the advantage that it is simple and safe to operate because it can be performed at or near atmospheric pressure. However, the concentration of the halogen or hydrogen halide that can be generated by this method is considerably low because of low decomposition efficiency and it is not easy to secure a sufficient concentration or amount for the subsequent use.